I'd like the post, but there's a way around this, besides finding a tune or working on one:
1) before you plan on buying a car, save your game in gt6 option menu (hit start).
2) Quit to XMB.
3) Plug a USB key formatted in FAT32 to your ps3, copy your gt6 save data to it (not mandatory, but it can come in handy in case you screw up)
4) Start gt6 again, go into dealerships, buy that car and test it on (various) difficult track(s), e.g. Nordschleife.
5) a) If you like it, keep on playing.
b) If you don't like the car, quit to XMB, delete your save file from your ps3, and restart the game. It will ask you if you want to load from the backup (which is your progress before you spend the $ on that crappy car, thanks to steps 1-4), say yes.
